Lubricating oil composition

ABSTRACT

The present invention provides a lubricating oil composition comprising a metal dithiophosphate and a poly(meth)acrylate containing a polar hydroxyl group in a base oil.

This invention provides lubricating oil compositions and in particulartransmission oils.

Since the transmissions in automotive vehicles are used under conditionsof high speed and high load, the transmission oils that are used thereinare required to have superior anti-seizing characteristics and anti-wearcharacteristics. Improvements in fuel consumption are also required. Theusual methods of improving fuel consumption are to reduce viscosity andto reduce friction by adding friction modifiers, but reducing frictionby adding friction modifiers gives rise to malfunctions in thesynchronising mechanisms in the case of manual transmissions, so thatthe method of improving fuel consumption by reducing viscosity hasbecome the main one.

However, reducing the viscosity of a lubricating oil gives rise toproblems whereby the seizing characteristics are reduced and damageoccurs in the machinery. Therefore, in order to increase theiranti-seizing characteristics, transmission oils in automotive vehiclesmake use of extra amounts of extreme-pressure additives such assulphur-type additives as typified by sulphurised olefins andsulphurised esters or phosphorus-type additives such as phosphate estersor amine salts of phosphate esters. See Japanese Laid-open Patent2004-262980, Japanese Laid-open Patent 10-259393 (1998), JapaneseLaid-open Patent 10-316987 (1998).

In recent years, because of a sudden increase in the demand forhigh-speed transport used on high-speed roads, the proportion ofhigh-speed running by automotive vehicles has increased, and at the sametime there has been a considerable rise in temperatures within enginesfor transmission oils used in manual gearboxes and reduction gears, astransmission gear units have been made more compact in response tomeasures to achieve higher engine outputs and improved aerodynamics.

To deal with this, it has been necessary to add extra amounts ofextreme-pressure agents such as the above mentioned sulphurised olefinsand phosphate esters, but there has been a strong tendency thereby fordeficiencies in oxidation stability and corrosive wear to be promoted athigh temperatures (normally 150° C. and above). Problems with corrosionof metals, especially copper, and sludge arise and as a result thephenomenon of wear and seizure being promoted on gear-tooth surfacesoccurs. Furthermore, in the case of manual transmissions, lowering ofthe friction coefficient or abnormal wear occurs in the synchroniserrings and gear cones within the synchromesh mechanisms, so that thesynchronisation does not work well and problems develop such asincreased effort needed to shift gears or even being unable to changegear.

Most recently, in order to resolve the above mentionedshift/synchronisation problems, there have been proposals to move fromthe above mentioned sulphur/phosphorus type extreme-pressure agents tothe metallic detergent/zinc dithiophosphate type. But problems to dowith not being able to achieve sufficient extreme-pressure performancehave been pointed out.

This invention is intended to obtain a lubricating oil compositionwhereby, even under high-speed and high-load operating conditions due tothe more compact size of transmissions, no anti-seizing performance islost, and yet there are adequate extreme-pressure qualities, fatiguecharacteristics are low, oxidation stability is high and longer life canbe achieved.

In order to resolve the above mentioned problems, this inventionprovides a lubricating oil composition by incorporating into a mineraloil and/or synthetic oil base oil a poly(meth)acrylate which contains apolar hydroxyl group, together with a metal dithiophosphate, hithertoregarded as having low extreme-pressure characteristics.

According to this invention, it is possible to obtain a lubricating oilcomposition wherein the anti-seizing characteristics are good, theextreme-pressure characteristics are the same as or better than withsulphur/phosphorus type extreme-pressure agents, the fatiguecharacteristics are low, oxidation stability is high and long life canbe achieved, and it is thus possible to obtain satisfactory lubricationperformance even under conditions of high-speed and high-load operationwhere transmissions have been made more compact.

There are no special restrictions on the base oil in the lubricating oilcomposition of this invention, and mineral oil type base oils and/orsynthetic type base oils as used in normal lubricating oils may be used.These mineral oil base oils and synthetic base oils may be used bymixing, in any proportions, suitable combinations of mineral oil baseoils, combinations of synthetic base oils, or mineral oil base oils andsynthetic base oils together.

There are no special restrictions on the viscosity index of thelubricating oil base oil, but the value thereof is preferably be atleast 90 (according to ASTM D2270) so as to obtain superior viscositycharacteristics from low to high temperatures, or more preferably atleast 100 and most preferably at least 110.

There are no special restrictions on the upper limit of the viscosityindex, and it is possible for it to be around 135 to 180 as in mineraloils such as the normal paraffin, slack wax and GTL wax type orisoparaffin type in which these are isomerised, or around 150 to 225 asin complex ester type base oils or HVI-PAO type base oils.

If the viscosity index of the lubricating oil base oil is less than 90,the amount of poly(meth)acrylate, which is added as a viscosity indeximprover, increases, and since poly(meth)acrylates of large molecularweight are used, the shear stability deteriorates, which is undesirable.

As specific illustration of mineral oil type base oils mention may bemade of those refined by subjecting lubricating oil fractions obtainedby vacuum distillation of atmospheric residues obtained by atmosphericdistillation of crude oil to treatments such as solvent deasphalting,solvent extraction of aromatics, hydrocracking, solvent dewaxing andhydrorefining, or wax-isomerised mineral oils.

There are no special restrictions on lubricating oil base oils differentfrom the above mentioned mineral oil base oils, and it is possible touse the synthetic oils used in the prior art. They may be selected ordesignated according, for instance, to application and may be used inany proportions with mineral oils. For example, mention may be made ofpoly-α-olefins, α-olefin co-polymers, polybutenes, polyol esters,dibasic esters, polyhydric alcohol esters, polyoxyalkylene glycols,polyoxyalkylene glycol esters, polyoxyalkylene glycol ethers, andcycloalkane compounds.

Further examples are lubricating oil base oils manufactured by themethod of isomerising GTL waxes (gas-to-liquid waxes) manufactured fromnatural gas by the Fischer-Tropsch process and so on.

The upper limit of the kinetic viscosity of the base oil at 100° C.(according to ASTM D445) is preferably 8 mm²/s and more preferably 6mm²/s. The lower limit of the kinetic viscosity of the base oil at 100°C. is preferably 2 mm²/s and more preferably 3 mm²/s. If the kineticviscosity at 100° C. is less than 2 mm²/s, there will be inadequateformation of an oil film at the lubricating points, so that lubricationwill deteriorate and there will be concern over seizing and wear oftooth surfaces. The evaporation losses of the base oil will alsoincrease, which is not desirable.

A metal dithiophosphate is incorporated in the base oil in thisinvention. Zinc dialkyl dithiophosphates and or molybdenum dialkyldithiophosphates and so on may be used for the metal dithiophosphate.Normally, zinc dialkyl dithiophosphates or molybdenum dialkyldithiophosphates having, as the alkyl groups therein, primary orsecondary alkyl groups having from 3 to 22 carbons or alkylaryl groupssubstituted by alkyl groups of from 3 to 18 carbons are used.

These zinc dialkyl dithiophosphates and molybdenum dialkyldithiophosphates may be used alone or in combinations of two or morethereof, but particularly preferred, as they increase the wearresistance, are those in which the main constituent is a zinc dialkyldithiophosphate with a secondary alkyl group.

As specific examples of the above mentioned zinc dialkyldithiophosphates, mention may be made of zinc dipropyl dithiophosphate,zinc dibutyl dithiophosphate, zinc dipentyl dithiophosphate, zincdihexyl dithiophosphate, zinc diisopentyl dithiophosphate, zincdiethylhexyl dithiophosphate, zinc dioctyl dithiophosphate, zinc dinonyldithiophosphate, zinc didecyl dithiophosphate, zinc didodecyldithiophosphate, zinc dipropylphenyl dithiophosphate, zincdipentylphenyl dithiophosphate, zinc dipropylmethylphenyldithiophosphate, zinc dinonylphenyl dithiophosphate and zincdidodecylphenyl dithiophosphate.

As specific examples of the above mentioned molybdenum dialkyldithiophosphates, mention may be made of molybdenum dipropyldithiophosphate, molybdenum dibutyl dithiophosphate, molybdenum dipentyldithiophosphate, molybdenum dihexyl dithiophosphate, molybdenumdiisopentyl dithiophosphate, molybdenum diethylhexyl dithiophosphate,molybdenum dioctyl dithiophosphate, molybdenum dinonyl dithiophosphate,molybdenum didecyl dithiophosphate, molybdenum didodecyldithiophosphate, molybdenum dipropylphenyl dithiophosphate, molybdenumdipentylphenyl dithiophosphate, molybdenum dipropylmethylphenyldithiophosphate, molybdenum dinonylphenyl dithiophosphate and molybdenumdidodecylphenyl dithiophosphate.

The zinc or molybdenum content in the lubricating oil composition in thecase of blends with these zinc dialkyl dithiophosphates and/ormolybdenum dialkyl dithiophosphates is approximately from 0.02 to 1.0wt. %, preferably from 0.04 to 0.5 wt. %, and more preferably from 0.08to 0.4 wt. %.

A poly(meth)acrylate having a hydroxyl group is incorporated in thelubricating oil composition together with the above mentioned metaldithiophosphate. This poly(meth)acrylate having a hydroxyl group is aco-polymer, and is a co-polymer which has as its essential constituentmonomers an alkyl(meth)acrylate having an alkyl group of from 1 to 20carbons and a vinyl monomer containing a hydroxyl group.

As specific examples of the above mentioned alkyl(meth)acrylate (a)having an alkyl group of 1 to 20 carbons, mention may be made of:

(a1) alkyl(meth)acrylates having an alkyl group of 1 to 4 carbons, forexample: methyl(meth)acrylate, ethyl(meth)acrylate, n- oriso-propyl(meth)acrylate and n-, iso- or sec-butyl(meth)acrylate;(a2) alkyl (meth)acrylates having an alkyl group of 8 to 20 carbons, forexample: n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-decyl(meth)acrylate, n-isodecyl (meth)acrylate, n-undecyl (meth)acrylate,n-dodecyl (meth)acrylate, 2-methylundecyl (meth)acrylate, n-tridecyl(meth)acrylate, 2-methyldodecyl (meth)acrylate, n-tetradecyl(meth)acrylate, 2-methyltridecyl (meth)acrylate, n-pentadecyl(meth)acrylate, 2-methyltetradecyl (meth)acrylate, n-hexadecyl(meth)acrylate and n-octadecyl (meth)acrylate, n-eicosyl (meth)acrylate,n-docosyl (meth)acrylate, (meth)acrylate of Dobanol 23 [mixture ofoxoalcohols of 12 carbons/13 carbons manufactured by Mitsubishi KaseiCorp. Ltd], and (meth)acrylate of Dobanol 45 [mixture of oxoalcohols of13 carbons/14 carbons manufactured by Mitsubishi Kasei Corp.];(a3) alkyl (meth)acrylates having an alkyl group of 5 to 7 carbons, forexample: n-pentyl (meth)acrylate and n-hexyl (meth)acrylate.

Of the above mentioned (a1) to (a3), the preferred substances are thosebelonging to (a1) and (a2), and more preferably the substances of (a2).Also, within (a1), those preferred from the standpoint of the viscosityindex are those with alkyl groups of 1 to 2 carbons. Within the abovementioned (a2), those preferred from the standpoint of solubility in thebase oil and low-temperature characteristics are those with alkyl groupsof from 10 to 20 carbons, and more preferably from 12 to 14 carbons.

The above mentioned vinyl monomer (b) containing a hydroxyl group andforming a co-polymer with the alkyl(meth)acrylate having an alkyl groupof from 1 to 20 carbons is a vinyl monomer containing in its moleculesone or more than one hydroxyl group (preferably one or two). As specificexamples, mention may be made of:

(b1) Hydroxyalkyl (2˜6 carbons) (meth)acrylates, for example:2-hydroxyethyl (meth)acrylate, 2- or 3-hydroxypropyl (meth)acrylate,2-hydroxybutyl (meth)acrylate and 1-methyl-2-hydroxyethyl(meth)acrylate;(b2) Mono- or di-hydroxyalkyl (1˜4 carbons) substituted(meth)acrylamides, for example: N,N-dihydroxymethyl (meth)acrylamide,N,N-dihydroxypropyl (meth)acrylamide and N,N-di-2-hydroxybutyl(meth)acrylamide;(b3) Vinyl alcohols (formed by hydrolysis of vinyl acetate units);(b4) Alkenols of from 3 to 12 carbons, for example: (meth)allyl alcohol,crotyl alcohol, isocrotyl alcohol, 1-octenol and 1-undecenol;(b5) Alkene diols of from 4 to 12 carbons, for example: 1-buten-3-ol,2-buten-1-ol and 2-buten-1,4-diol;(b6) Hydroxyalkyl (from 1 to 6 carbons) alkenyl (3˜10 carbons) ethers,for example: 2-hydroxyethylpropenyl ether;(b7) Aromatic monomers containing a hydroxyl group, for example: o-, m-or p-hydroxystyrene;(b8) Polyhydric (from trihydric to octahydric) alcohols, for example:alkane polyols, intramolecular or intermolecular dehydrates thereof,alkenyl (3˜10 carbons) ethers of sugars (e.g. glycerine,pentaerythritol, sorbitol, sorbitan, diglycerine, sucrose) or(meth)acrylates of sugars (e.g. sucrose (meth)acryl ether);(b9) polyoxyakylene chains and vinyl monomers containing hydroxylgroups, for example: mono(meth)acrylates or mono(meth)allyl ethers ofpolyoxyalkylene glycols (alkylene group of from 2 to 4 carbons, degreeof polymerisation from 2 to 50) or polyoxyalkylene polyols{polyoxyalkylene ethers (alkyl groups of from 2 to 4 carbons, degree ofpolymerisation from 2 to 100) of the above mentioned trihydric tooctahydric alcohols} {e.g. polyethylene glycol (degree of polymerisationfrom 2 to 9) mono(meth)acrylates, polypropylene glycol (degree ofpolymerisation from 2 to 30) mono(meth)allyl ether}.

Of the above mentioned (b1) to (b9), from the standpoint of effect ofimproving the viscosity index the preferred type is (b1), and2-hydroxy-ethyl methacrylate in particular.

The respective proportions in the monomer constituting the abovementioned poly(meth)acrylate co-polymer containing a hydroxyl group are,from the standpoint of the viscosity index, preferably as follows.

The lower limit of the above mentioned constituent (a) is preferably 50wt. % and more preferably 75 wt. %, and the upper limit is preferably 95wt. % and more preferably 85 wt. %.

The lower limit of the above mentioned (a1) is preferably 0 wt. % andmore preferably 1 wt. %, and the upper limit is preferably 20 wt. % andmore preferably 10 wt. %.

The lower limit of the above mentioned (a2) is preferably 50 wt. % andmore preferably 70 wt. %, and the upper limit is preferably 95 wt. % andmore preferably 90 wt. %.

The lower limit of the above mentioned (b) is preferably 5 wt. % andmore preferably 7 wt. %, but particularly preferable is 11 wt. %, andthe upper limit is preferably 50 wt. % and more preferably 30 wt. %, butparticularly preferable is 15 wt. %.

The lower limit of the sum of the above mentioned (a)+(b) is preferably55 wt. % and more preferably 82 wt. %, and the upper limit is preferably100 wt. %.

Other monomers may be co-polymerised together with the above mentioned(a) and (b) in the above mentioned poly(meth)acrylate co-polymercontaining a hydroxyl group. Such monomers include monomers containingnitrogen atoms (c). Specific examples include:

(c1) Monomers containing a nitro group, for example 4-nitrostyrene;(c2) Vinyl monomers containing primary ˜tertiary amines, such as:(c2-1) Vinyl monomers containing primary amines, for example: 3˜6-carbonalkenyl amine [(meth)allylamine, crotylamine, and so on], aminoalkyl(2˜6 carbons) (meth)acrylates [aminoethyl (meth)acrylate and so on];(c2-2) Vinyl monomers containing secondary amines, for example: alkyl(1˜6 carbons) aminoalkyl (2˜6 carbons) (meth)acrylates[t-butylaminoethyl methacrylate, methylaminoethyl (meth)acrylate, and soon], diphenylamine (meth)acrylamides [4-diphenylamine (meth)acrylamide,2-diphenylamine (meth)acrylamide, and so on], and dialkenylamines with6˜12 carbons [di(meth)allyl amine and so on];(c2-3) Vinyl monomers containing tertiary amines, for example: dialkyl(1˜4 carbons) aminoalkyl (2˜6 carbons) (meth)acrylates[dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate,and so on], dialkyl (1˜4 carbons) aminoalkyl (2˜6 carbons)(meth)acrylamides [dimethylaminoethyl (meth)acrylamide,diethylaminoethyl (meth)acrylamide, dimethylaminopropyl(meth)acrylamide, and so on], and aromatic vinyl monomers containingtertiary amino groups [N,N-dimethylaminostyrene and so on];(c2-4) Vinyl monomers containing complex rings containing nitrogen[morpholinoethyl (meth)acrylate, 4-vinylpyridine, 2-vinylpyridine,N-vinylpyrrole, N-vinylpyrrolidone, N-vinylthiopyrrolidone, and so on];(c3) Amphoteric vinyl monomers, for example: N-(meth)acryloyloxy (oramino) alkyl (1˜10 carbons) N,N-dialkyl (1˜5 carbons) ammonium-N-alkyl(1˜5 carbons) carboxylates (or sulphates), e.g. N-(meth)acryloyloxyethylN,N-dimethylammonium N-methyl carboxylate, and N-(meth)acryloyloxyethylN,N-dimethylammonium propyl sulphate;(c4) monomers containing nitrile groups, for example: acrylonitrile.

Such monomers also include aliphatic hydrocarbon vinyl monomers (d), forexample: alkenes of 2˜20 carbons [ethylene, propylene, butene,isobutylene, pentene, heptene, diisobutylene, octene, dodecene,octadecene, and so on] and alkadienes of 4˜12 carbons [butadiene,isoprene, 1,4-pentadiene, 1,6-heptadiene, 1,7-octadiene, and so on].

Further, there are alicyclic hydrocarbon vinyl monomers (e), forexample: cyclohexene, (di)cyclopentadiene, pinene, limonene, indene,vinylcyclohexene and ethylidenebicycloheptene.

There are also aromatic hydrocarbon vinyl monomers (f), for example:styrene, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene,4-ethylstyrene, 4-isopropylstyrene, 4-butylstyrene, 4-phenylstyrene,4-cyclohexylstyrene, 4-benzylstyrene, 4-crotylbenzene and2-vinylnaphthalene.

And there are vinyl esters, vinyl ethers, vinyl ketones (g), forexample: vinyl esters of saturated fatty acids of 2˜12 carbons [vinylacetate, vinyl propionate, vinyl butyrate, vinyl octanoate, and so on],vinyl ethers of alkyls, aryls or alkoxyalkyls of 1˜12 carbons[methylvinyl ether, ethylvinyl ether, propylvinyl ether, butylvinylether, 2-ethylhexylvinyl ether, phenylvinyl ether, vinyl 2-methoxyethylether, vinyl 2-butoxyethyl ether, and so on], and vinyl ketones ofalkyls or aryls of 1˜8 carbons [methylvinylketone, ethylvinylketone,phenylvinylketone].

Further, there are esters of unsaturated polycarboxylic acids (h). Forexample mention may be made of alkyl, cycloalkyl or aralkyl esters ofunsaturated polycarboxylic acids, and these include alkyl diesters ofunsaturated dicarboxylic acids [maleic acid, fumaric acid, itaconicacid, and so on] with 1˜8 carbons [dimethyl maleate, dimethyl fumarate,diethyl maleate, dioctyl maleate].

It is also possible to use vinyl monomers containing polyoxyalkylenechains (those not containing hydroxyl groups) (i), for example:mono(meth)acrylates of monoalkyl (1˜18 carbons) ethers ofpolyoxyalkylene glycols (alkylene groups of 2˜4 carbons, degree ofpolymerisation 2˜50) or polyoxyalkylene polyols [polyoxyalkylene ethersof the above mentioned trihydric to octahydric alcohols (alkyl groups of2˜4 carbons, degree of polymerisation 2˜100)] [for example,methoxypolyethylene glycol (molecular weight 110˜310) (meth)acrylate andlauryl alcohol ethylene oxide adduct (2˜30 mol) (meth)acrylate].

It is possible to use vinyl monomers containing carboxyl groups (j), forexample: vinyl monomers containing monocarboxyl groups such asunsaturated monocarboxylic acids [(meth)acrylic acid, α-methyl(meth)acrylic acid, crotonic acid, cinnamic acid, and so on], monoalkyl(1˜8 carbons) esters of unsaturated dicarboxylic acids [monoalkyl estersof maleic acid, monoalkyl esters of fumaric acid, monoalkyl esters ofitaconic acid, and so on]; it is also possible to use copolymers of twoor more vinyl monomers containing carboxyl groups, for example maleic,acid, fumaric acid, itaconic acid and citraconic acid.

Of the above mentioned additional monomers (c), (d), (e), (f), (g), (h),(i) and (j), those preferred are (c), and of (c) it is possible to usetwo or more. Of the above mentioned (c), those preferred are (c2), andeven more preferred are dimethylaminoethyl (meth)acrylate anddiethylaminoethyl (meth)acrylate.

The lower limit of the weight average molecular weight of thepoly(meth)acrylate containing a hydroxyl group is preferably 5,000 andmore preferably 8,000, while 10,000 is especially preferred. The upperlimit is preferably 50,000 and more preferably 40,000, while 35,000 isespecially preferred and 30,000 is even more especially preferred.

If the weight average molecular weight is within the above mentionedrange, good shear stability can be imparted. This weight averagemolecular weight is according to gel permeation chromatography, and isobtained by conversion to polystyrene.

The above mentioned weight average molecular weights may be adjusted onthe basis of the temperature at the time of polymerisation, monomerdensity (solvent concentration), amount of catalyst or amount of chaintransfer agent.

The polydispersity (Mw/Mn) of this poly(meth)acrylate containing ahydroxyl group is preferably 1.0˜2.5, and more preferably 1.2˜2.0, butespecially preferred is 1.5˜1.7. With a small polydispersity, the shearstability is good. Mn is obtained in the same way as Mw.

Also, the lower limit of the solubility parameter thereof is preferably8.6, and more preferably 9.2, but especially preferred is 9.3, while theupper limit is preferably 11, and more preferably 10.5, but especiallypreferred is 9.7. This solubility parameter value was calculated by themethod of Fedors (Polym. Eng. Sci. 14 (2), 152 (1974)).

Further, the HLB of the poly(meth)acrylate containing a hydroxyl groupis preferably 0.5˜7. If the HLB is within this range, thedemulsification characteristics are especially good. More preferable isa HLB of 1˜6.5, and especially preferred is 1.5˜6. This HLB value iscalculated by Oda's method, which is based on organic and inorganicconcepts (“New Introduction to Surfactants”, published by Sanyo KaseiKogyo Co. Ltd., page 128).

The hydroxyl number of the poly(meth)acrylate containing a hydroxylgroup, used as an additive, is 10˜100, preferably 20˜50, and morepreferably 25˜35. The measurement of the hydroxyl number is the valueobtained by measuring in accordance with JIS K3342 (1961). It shows theamount of hydroxyl group in the additive.

It is also possible to add the normal type of poly(meth)acrylates of theprior art which do not contain a hydroxyl group at the same time as thepoly(meth)acrylate containing a hydroxyl group. The amount thereof isencompassed by the ratio poly(meth)acrylate containing a hydroxyl group:poly(meth)acrylate not containing a hydroxyl group being 100:0˜40:60.

It is also possible to add, as required, suitable amounts of otheradditives such as rust preventatives, detergents, dispersants,anti-oxidants, extreme-pressure agents, oiliness agents, frictionmodifiers, pour-point depressants and defoaming agents.

EXAMPLES

The following test materials were prepared to produce Examples ofEmbodiment 1 to 6 and Comparative Examples 1 to 5.

(1) Base oils(1-1) Base Oil A: the oil type was a mixture of Group I, Group II andGroup III oils (characteristics: kinetic viscosity at 40° C., 18.7mm²/s; kinetic viscosity at 100° C., 4.1 mm²/s; viscosity index, 122);(1-2) Base Oil B: the oil type was a mixture of Group I, Group II andGroup V oils (characteristics: kinetic viscosity at 40° C., 16.5 mm²/s;kinetic viscosity at 100° C., 3.6 mm²/s; viscosity index, 96).(2) Poly(meth)acrylate (PMA) containing a hydroxyl group: weight averagemolecular weight (Mw), 16,000; hydroxyl number 30 (Aclube V1070manufactured by Sanyo Kasei Kogyo Co. Ltd.).(3) Poly(meth)acrylates (PMA) of the conventional type not containing ahydroxyl group(3-1) Conventional PMA (I): weight average molecular weight, 15,000(Viscoplex 0-050 manufactured by Degussa);(3-2) Conventional PMA (II): weight average molecular weight, 35,000(Aclube 811 manufactured by Sanyo Kasei Kogyo Co. Ltd.);(3-3) Conventional PMA (III): weight average molecular weight, 75,000(Viscoplex 0-291 manufactured by Degussa).(4) Zinc dialkyl dithiophosphates(4-1) ZnDTP (I): Primary-type zinc diethylhexyl dithiophosphate;(4-2) ZnDTP (II): Secondary-type zinc dialkyl dithiophosphate with 3˜6carbons.(5) Phenolic anti-oxidant: manufactured by Ciba Specialty Chemicals,Irganox-L135, benzenepropanoic acid,3,5-bis(1,1-dimethyl-ethyl)-4-hydroxy, C7-C9 side chain alkyl ester.(6) Sulphurised fat/oil: sulphurised lard oil(7) Phosphate ester: alkylated triphenyl phosphate(8) Packaged additives: detergent+dispersant+anti-oxidant.

The lubricating oil compositions for Examples 1 to 6 and ComparativeExamples 1 to 4 that were used to compare therewith were obtained byusing the above mentioned test materials and thoroughly mixing them onthe basis of the compositions shown in Tables 1 and 2. A commercialtransmission oil was used for Comparative Example 5.

Tests

The following tests were carried out in order to compare theperformances of the examples and the comparative examples.

Tests to Measure Characteristics and Composition

1. 40° C. kinetic viscosity (ASTM D445; units: mm²/s)2. 100° C. kinetic viscosity (ASTM D445; units: mm²/s)3. Viscosity index (VI; ASTM D2270)4. Phosphorus content (units: % by weight)5. Sulphur content (units: % by weight)

Falex Seizure Test

Test conditions: The test pieces were set up in the test rig and an oilbath containing 60 ml of test oil was heated by means of a heater untilthe oil temperature reached 100° C.

When the oil temperature had reached 100° C., the test rig drive motorwas actuated and the test began with a load on the block of 136.08 kg(300 pounds). After running for 2 minutes at 136.08 kg (300 pounds), theload was increased to 226.8 kg (500 pounds), running for 1 minute.Thereafter, the load was increased by 136.08 kg (300 pounds) a time,running for 1 minute, this being repeated until there was an occurrenceof seizing.

Test pieces: (1) Falex #8 test pin

(2) Falex Xee block [standard]

Evaluation: Assessment of the occurrence of seizing was based on theoccurrence of a sudden rise in friction torque and the abnormalscreeching noise, or on rupture of the pin.

The heater was stopped when the temperature reached 100° C., and thetest was continued in that state.

Higher values show superior anti-seizing properties.

ISOT test

Test specimens on which an oxidation test had been carried out underconditions of 135±0.5° C.×96 hr in accordance with JIS K2514 werecompared with new oil, and the rate of change in kinetic viscosity at40° C. (increase rate) and rate of change in kinetic viscosity at 100°C. (increase rate) were measured. A small increase rate is best and thepass value was set at 8 or below.

Sonic Test

Under the test conditions specified in JASO M347-95, a 30 ml test samplewas exposed to ultrasonic waves for 1 hour, and the rate of change inkinetic viscosity at 40° C. (reduction rate) and rate of change inkinetic viscosity at 100° C. (reduction rate) were measured with newoil. A small reduction rate is best and the pass value was set at 7 orbelow.

FZG Pitting Test

In accordance with DIN 51354. Type: PT-C gears. Performed with load: 9stages, oil temperature: 120° C., speed: 1440 revolutions/min.

The method of assessing fatigue life was to stop the test rig and carryout periodic inspections of the tooth surfaces, and to reckon it as thetime (hr) when 1 mm² of pitting had occurred. The inspection intervalswere every 8 hours in the initial 24 hours and then every 2˜4 hours.

Results

The results of the above mentioned tests are shown in Tables 1 and 2.

Discussion

In the case of Examples 1˜6, good results were obtained for anti-seizingproperties on the basis that all were 907.2 kg (2000 pounds) or higherin the Falex seizing test. Long fatigue lives of 42˜75 hours wereobtained, too, in the FZG pitting tests. Good results at a steady levelwere also obtained in the ISOT and Sonic tests.

Comparative Examples 1, 2 and 4 used zinc dipropylphenyldithiophosphate, but used a conventional type of PMA which did notcontain a hydroxyl group. Although Comparative Example 1 gave goodresults in the ISOT and Sonic tests, it was inferior as regardsanti-seizing properties to the examples of embodiment, scoring 567 kg(=1250 pounds) in the Falex seizing test. Comparative Examples 2 and 4gave good results in the ISOT test, but were inferior as regardsanti-seizing properties to the examples of embodiment in the Falexseizing test.

Comparative Example 3 contained a PMA containing a hydroxyl group and aconventional type of PMA not containing a hydroxyl group, but it did notincorporate ZnDTP. Instead, it contained an SP type additive, and areasonable enough result for anti-seizing properties was obtained with793.8 kg (=1750 pounds) in the Falex seizing test, but a satisfactoryresult was not achieved as regards oxidation stability in the ISOT test.

Also, in the FZG pitting test, Comparative Example 1 scored 42 hours butin the case of Comparative Examples 2 and 4 the fatigue life was short,at 30˜32 hours. Comparative Example 3 had bad results in the ISOT testand so the FZG pitting test was omitted.

The commercial transmission oil of Comparative Example 5 had poorresults in the Sonic test, and did not give good results either in theFalex seizing test and FZG pitting test.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Blend Base Oil A BalanceBalance Balance Balance Balance Base Oil B Balance PMA containinghydroxyl 10 10 10 14 10 14 group (16000 MW) Conventional PMA (I) (15000MW) Conventional PMA (II) 14 (35000 MW) Conventional PMA (III) 2 2 2 2(75000 MW) Zn DTP (I) 1.7 Zn DTP (II) 1.7 3.4 1.7 1.7 1.7 Phenolicanti-oxidant 0.3 0.3 0.3 0.3 0.3 0.3 Sulphurised fat/oil 0.5(sulphurised lard oil) Phosphate ester 0.1 Packaged additives 4.5 4.54.5 4.5 4.5 4.5 TOTAL 100 100 100 100 100 100 Tests Characteristics &composition of new oil * Kinetic viscosity (40° C.) 31.65 30.13 31.6572.11 35.21 30.85 * Kinetic viscosity (100° C.) 6.734 6.514 6.632 13.767.072 6.435 * Viscosity index (VI) 178 177 172 198 168 168 * Phosphoruscontent (wt. %) 0.16 0.16 0.32 0.16 0.17 0.16 * Sulphur content (wt. %)0.5 0.5 0.8 0.5 0.6 0.5 Falex (pounds) 2250 2000 2000 2000 2000 2250ISOT (40° C. viscosity increase rate) 4.1 4.5 4 6.9 5.8 5.5 ISOT (100°C. viscosity increase rate) 4.6 4.9 4.3 7.3 6.2 5.7 Sonic (40° C.viscosity reduction rate) 4.9 4.9 4.8 4.1 4.8 0.6 Sonic (100° C.viscosity reduction rate) 5 5.1 5.1 3.9 5.3 0.7 FZG pitting test (hr) 7562 48 60 42 68

TABLE 2 Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5Blend Base Oil A Balance Balance Balance Commercial Base Oil B Balancemulti oil PMA containing hydroxyl 10 (75W-85) group (16000 MW)Conventional PMA (I) 10 (15000 MW) Conventional PMA (II) 24 (35000 MW)Conventional PMA (III) 5 2 (75000 MW) Zn DTP (I) Zn DTP (II) 1.7 1.7 1.7Phenolic anti-oxidant 0.3 0.3 0.3 Sulphurised fat/oil 1.5 (sulphurisedlard oil) Phosphate ester 1.5 Packaged additives 4.5 4.5 4.5 4.5 TOTAL100 100 100 100 100 Tests Characteristics & composition of new oil *Kinetic viscosity (40° C.) 35.48 29.97 35.32 72.8 * Kinetic viscosity(100° C.) 7.081 6.529 7.129 14.1 * Viscosity index (VI) 167 181 170202 * Phosphorus content (wt. %) 0.16 0.16 0.16 0.16 * Sulphur content(wt. %) 0.5 0.5 0.4 0.5 Falex (pounds) 1250 1250 1750 1250 1250 ISOT(40° C. viscosity increase rate) 1.5 1.2 10.8 2.8 8.2 ISOT (100° C.viscosity increase rate) 1.6 1.8 11.2 3.2 5.9 Sonic (40° C. viscosityreduction rate) 1.3 6.3 4.9 7.8 17.4 Sonic (100° C. viscosity reductionrate) 1.5 6.2 5 8 16.9 FZG pitting test (hr) 42 30 No data 32 32

1-10. (canceled)
 11. A lubricating oil composition comprising a baseoil, a poly(meth)acrylate containing a hydroxyl group, and a metaldithiophosphate.
 12. The lubricating oil composition of claim 11,wherein the poly(meth)acrylate containing a hydroxyl group has anaverage molecular weight that is from 5,000 to 50,000.
 13. Thelubricating oil composition of claim 11 further comprising apoly(meth)acrylate that does not contain a hydroxyl group.
 14. Thelubricating oil composition of claim 13, wherein the poly(meth)acrylatethat does not contain a hydroxyl group has an average molecular weightthat is from 20,000 to 100,000.
 15. The lubricating oil composition ofclaim 13 having a weight ratio of the poly(meth)acrylate containing ahydroxyl group to the poly(meth)acrylate that does not contain ahydroxyl group in the range of 100:0 to 40:60.
 16. The lubricating oilcomposition of claim 11 further comprising a sulphurised fat or oil anda phosphate ester.
 17. The lubricating oil composition of claim 11,wherein the base oil has a kinetic viscosity at 100° C. of at least 2mm²/s and a viscosity index of at least
 90. 18. The lubricating oilcomposition of claim 11, wherein the metal dithiophosphate is a zincdithiophosphate.
 19. A lubricating oil composition comprising: (a) abase oil having a kinetic viscosity at 100° C. of at least 2 mm²/s andno greater than 8 mm²/s and a viscosity index of at least 90; (b) apoly(meth)acrylate containing a hydroxyl group, wherein thepoly(meth)acrylate containing a hydroxyl group is a co-polymer thatcomprises (i) at least 50 weight percent of an alkyl(meth)acrylatemonomer having an alkyl group of from 1 to 20 carbons and (ii) at least5 weight percent of a vinyl monomer having an alkyl group of from 1 to20 carbons; and (c) one or more of zinc dithiophosphates at a combinedconcentration such that the total content of zinc is approximately from0.02 to 1.0 weight percent of the lubricating oil composition.
 20. Thelubricating oil composition of claim 19, wherein the alkyl group of thealkyl(meth)acrylate monomer has from 8 to 20 carbons, the vinyl monomeris a hydroxyalkyl (2-6 carbons)(meth)acrylate, and the zincdithiophosphates are zinc dialkyl dithiophosphates having, as the alkylgroups therein, secondary alkyl groups of from 3 to 22 carbons.
 21. Amethod of lubricating an apparatus, the method comprising lubricatingthe apparatus with a lubricating oil composition that comprises a baseoil, a poly(meth)acrylate containing a hydroxyl group, and a metaldithiophosphate.
 22. The method of claim 21, wherein thepoly(meth)acrylate containing a hydroxyl group has an average molecularweight that is from 5,000 to 50,000.
 23. The method of claim 21, whereinthe lubricating oil composition further comprises a poly(meth)acrylatethat does not contain a hydroxyl group.
 24. The method of claim 23,wherein the poly(meth)acrylate that does not contain a hydroxyl grouphas an average molecular weight that is from 20,000 to 100,000.
 25. Themethod of claim 23, wherein the lubricating oil composition has a weightratio of the poly(meth)acrylate containing a hydroxyl group to thepoly(meth)acrylate that does not contain a hydroxyl group in the rangeof 100:0 to 40:60.
 26. The method of claim 21, wherein the lubricatingoil composition further comprises a sulphurised fat or oil and aphosphate ester.
 27. The method of claim 21, wherein the base oil has akinetic viscosity at 100° C. of at least 2 mm²/s and a viscosity indexof at least
 90. 28. The method of claim 21, wherein the metaldithiophosphate is a zinc dithiophosphate.